1. Field of the Invention
The present invention relates to an adsorber to capture and enrich carbon dioxide (CO2) from flue gas and having a unique moving bed structure in which regeneration occurs by heating the adsorbent. This type of system is commonly referred to as temperature swing adsorption (TSA), since the adsorbent temperature is cycled synchronously during uptake and release. This particular type of TSA concept involves a moving bed of adsorbent. It offers significant advantages such as, for example, higher efficiency, lower thermal mass, lower heat loss, and lower cost than fixed bed (stationary adsorbent) systems employed to achieve the same end. This type of adsorber is especially well suited to applications where flue gas contains at least 3% carbon dioxide, and at least 1,000 standard cubic feet per hour of flue gas.
The present invention also relates to the removal of certain components from gas streams used in industrial applications, such as, for example, air containing SO2, natural gas or landfill gas containing excess CO2, air-drying, and separation of hydrocarbon mixtures.
2. Discussion of the Background
One problem of our modern society is energy production from combustion of fossil fuels, and the associated emissions. Though there remains some controversy, the two key problems associated with such emissions are acid rain and global warming.
For example, the presence of sulfur in some coal deposits leads to emissions of sulfur oxides (SOx) with SO2 being chief among them. Since the 1960s, SOx has been recognized as a contributor to so-called acid rain, which was blamed for devastation of forests, lakes, and agricultural output (http://www.epa.gov/airmarkets/acidrain/index.html, Dec. 17, 2004). Fortunately, measures have been taken to prevent SOX from reaching the environment.
In the 1990s, global warming, or more broadly, climate change, became recognized as a serious potential problem (http://yosemite.epa.gov/oar/globalwarming.nsf/content/index.html, Dec. 17, 2004). CO2 is produced by combustion of fossil fuels, including coal and natural gas, and other hydrocarbon fuels including propane, liquefied petroleum gas (LPG), heating oil, landfill gas, gasoline, jet fuel, diesel fuel, and naphtha. CO2 is called a greenhouse gas because, compared with the main constituents of air, it tends to admit solar energy but restricts heat loss from the surface of the earth.
Accordingly, many individuals, organizations, and even countries, feel CO2 is mostly responsible for global warming or climate change, and they want to limit emissions of CO2 into the atmosphere. At present, there is no economical means to collect CO2 emissions from power plants or other point sources. The present invention pertains to capturing CO2 from flue gas prior to its discharge into the atmosphere, though it applies to other gas separation applications, as well.
Coupled with the perceived problem of climate change is the gradual depletion of fossil energy sources, such as, for example, crude oil. Consequently, various techniques have been developed to enhance the recovery of crude oil from geologic reservoirs. One of the more promising enhanced crude oil recovery techniques is the injection of CO2 into crude oil reservoirs that have been partly depleted using conventional primary techniques. This oil recovery technique is described in U.S. Pat. No. 3,442,332, and in other references. An inexpensive means to recover CO2 from flue gas will improve the economics for extracting crude oil from existing reservoirs.
3. Background on Moving Bed Adsorbers
The most well known type of moving bed adsorber is used for capturing volatile organic compounds (VOCs) from air. Berg in U.S. Pat. No. 2,519,873; Murakami and Okamoto in U.S. Pat. No. 4,047,906; Jacquish in U.S. Pat. No. 4,869,3734; Dingfors in U.S. Pat. No. 4,902,311; Cioffi and Cowles in U.S. Pat. No. 5,676,738; and Vickery in U.S. Pat. No. 6,027,550, describe examples.
The Berg patent teaches separation of a gaseous mixture by selective adsorption, using an apparatus having an adsorption section and a stripping section. This patent was the basis of the so-called Hypersorber, used for fractionating hydrocarbon gases with activated carbon. The stripping section has a contacting part and a heating part, with regeneration occurring by combined action of heat and stripping gas (e.g., steam). There also is an elevator to convey regenerated adsorbent from the bottom of the stripping section to the top of the adsorption section. As reported by Treybal “Several adsorbers on a very large scale were built, but the very brittle carbon was subject to serious attrition losses, and no new continuous-flow, countercurrent device for plug flow of solids and gas is believed to be in operation.” (Treybal, R. E., “Mass-Transfer Operations,” 3rd Ed., McGraw-Hill, New York, 1980). Additional information was reviewed by Wankat (Wankat, P. C., “Large Scale Adsorption and Chromatography,” Vol. II, CRC Press, Boca Raton, 1986) who mentioned that, “Attrition losses were a problem, but could be reduced if modern spherical carbon beads were used.”
The Murakami and Okamoto patent discloses an apparatus for purifying a waste gas containing pollutants. The apparatus is a tower comprised of an adsorbing section, containing trays with weirs, dividing each tray into two zones, and serving to regulate the lateral flow of adsorbent across the tray. Both zones are perforated, but the holes in one zone are too small for the adsorbent to penetrate, while the other zone allows adsorbent to fall to the tray below, into a zone through which it cannot pass. The gas was fed to the tower below the trays, and the adsorbent was fed to the top tray of the tower, resulting in overall countercurrent flow, though the adsorbent on any tray would be in cross-flow.
The Jacquish patent shows an adsorption system for treating air that is contaminated with solvent vapors (i.e., VOCs). The adsorption section contains parallel passages made of screen, through which the adsorbent falls, while the contaminated gas flows horizontally, which causes cross-flow between the gas and adsorbent. The adsorbent is collected from the parallel passages and transferred by gravity downwards to a desorber, where the VOCs are desorbed into a carrier gas, e.g., nitrogen. That gas is split and some of which flows vertically downwards in the same direction as the adsorbent, while the rest flows upwards more or less counter to the adsorbent. The net effect is cross-flow. The adsorbent is transferred via a conveyor to the top of the adsorption section.
The Dingfors patent teaches adsorption using a fluidized bed of macroporous polymeric particles through which passes air that is contaminated with solvent vapors (i.e., VOCs). The polymeric particles adsorb the solvent vapors, and are transported to a free-standing stripper (desorber), which effects desorption of the solvent by application of hot air, in countercurrent flow, while passing through a heat exchanger. The desorbed solvent vapors and air are cooled to condense the solvents for reuse.
The Cioffi and Cowles patent reveals another VOC recovery system in which the contaminated gas flows upwards and the adsorbent flows downwards, counter to the gas path. The adsorption section contains 1 to 20 sieve trays (perforated plates), which allow gas to flow upwards (through the perforations) and passageways (downcomers), which allow the adsorbent to pass downwards to the tray below. The adsorbent is transferred to the top of a free-standing desorber, where the VOCs are desorbed into a carrier gas, which flows counter to the solid, i.e., flowing upwards. The adsorbent is transferred pneumatically.
The Vickery patent discloses another VOC recovery system in which the contaminated gas flows upwards and the adsorbent flows downwards, counter to the gas path. The adsorption section contains two regions, which allow the adsorbent to be regenerated in separate, freestanding desorbers. Each adsorption region contains trays with weirs, which serving to regulate the lateral flow of adsorbent across the tray, and to the tray below. The trays are perforated, but the holes are too small for the adsorbent to penetrate. After passing through an adsorption region, the adsorbent is transferred to a free-standing desorber, where the VOCs are desorbed into a carrier gas, which flows to a freestanding thermal oxidizer. The adsorbent is transferred pneumatically back to the adsorption section.
D. Aaron and C. Tsouris from Oak Ridge National Laboratory recently published paper, “Separation of CO2 from Flue Gas: A Review,” Separation Science and Technology, Vol 40, pp 321-348 (2005). The abstract states, “Upon completion of this review, it was concluded that the most promising current method for CO2 separation is liquid absorption using monoethanolamine (MEA).” It goes on to say that certain membrane processes might be appealing, “potentially more efficient at separation than liquid absorption,” and that other methods [e.g., adsorption] “are either too new for comparison or appear unlikely to experience significant changes to make them desirable for implementation.”
4. Background on Other Carbon Dioxide Capture Technologies
Capture technologies can be divided into two broad categories: post-combustion capture technologies (so called end-of-pipe capture Of CO2 from flue gases), and pre-combustion capture technologies (CO2 capture by fuel conversion via chemical reactions). The first category includes absorption (e.g., with mono-ethanol-amine), adsorption (either pressure swing or temperature swing), and membrane separation. The second category includes coal gasification, i.e., by partial oxidation, which produces syngas: mostly carbon monoxide (CO) and hydrogen (H2). The CO and H2 then are separated and combusted in a controlled environment releasing almost pure CO2 and H2O. Alternatively, the carbon can be removed as the syngas is formed, via carbonation of metal oxides such as calcium, magnesium, or others, in order to produce hydrogen. Another pre-combustion approach is called oxygen combustion capture (or sometimes called oxyfuel), which involves separation of air (to remove nitrogen) in order to obtain relatively pure oxygen (O2), which is mixed with recycled CO2 to avoid excessive temperature.
When the Department of Energy considers the hypothetical question, “What capture technology can be used at my local power plant?”, the answer is: “In the future, emerging R&D will provide numerous cost effectives technologies for capturing carbon dioxide from power plants. At present, however, state-of-the-art technologies for existing power plants are essentially limited to ‘amine absorbents’.” (http://www.netl.doe.gov/coal/Carbon%20Sequestration/Resources/faqs.html, Jan. 5, 2005). That source goes on to explain the basic concept of absorption: “The process works as follows. Flue gas that would normally go out the stack is bubbled through a solution of water and amines. The amines in the water react with the carbon dioxide in the flue gas to form an intermediate chemical called a rich amine. The rich amine is soluble and stays in the water solution. Some of the flue gas bubbles out of the top of the amine solution and is emitted to the air just like the flue gas was before, but a portion of the carbon dioxide has reacted with the amines and remains in solution. The rich amines are pumped to another vessel where they are heated to make them decompose back into regular (lean) amines and carbon dioxide gas. The pure carbon dioxide gas is collected from this vessel and the regular amines are recycled to the flue contactor gas vessel.”
Any of these other technologies that rely on compression or evacuation (e.g., pressure swing adsorption, membrane processes, and some versions of absorption), are hindered by the inherent cost of that operation. An illustration of the inherent cost, which is unavoidable for is the power requirement.
                    Power        =                              γ                          γ              -              1                                ⁢                                    QRT              η                        [                                                            (                                                            P                      H                                                              P                      L                                                        )                                                                      γ                    -                    1                                    γ                                            -              1                        ]                                              (        1        )            If the flue gas must be compressed in order to treat it (e.g., via a membrane unit), the power cost will depend on the required pressure. For example, if CO2 is collected from a cement plant, at a effluent mole fraction of 0.1478, and an overall flow rate of 243.1 thousand standard cubic feet per minute (corresponding to an emission rate of 3,000 tons of CO2 per day), and if the pressure required is 44.1 psig (starting at atmospheric pressure), the power required would be about 30 MW. If power costs $0.05 per kWh, the cost would be about $11.86 per ton of CO2 captured. Likewise, if vacuum must be used to collect the concentrated CO2, the cost will depend on the extent of evacuation. For example, for the same cement plant and basic power cost, if only the 3,000 tons of CO2 per day were collected at 1.0 psia and compressed only to atmospheric pressure, the power required would be about 10 MW. The cost per ton of CO2 would be about $4.17. Note that for both of these illustrations, the cost cited only represents the cost of the power, not the cost of the equipment to pump the gas, nor the cost of the device to perform the separation.
5. Adsorbent Selection
The most important attributes of an adsorbent for any application are: working capacity (change in loading of the desired strongly adsorbed component(s) between the uptake step and release step, as shown in FIG. 2), selectivity (ability to adsorb the desired strongly adsorbed component(s) and not to adsorb other components that are not desired), kinetics (speed of uptake and release of the desired strongly adsorbed component(s)), durability (ability to withstand the stresses in a moving bed adsorption system over many circuits or cycles), compatibility (suitable inertness, i.e., resistance to degradation or poisoning by contaminants in the feed mixture, and to decomposition of those or other contaminants), and cost (i.e., suitably low in order that the entire process is economical). The overall performance and economic benefits of the process depend on all of these.
Suitable adsorbents for this application are those having reasonably large working capacity over the relevant temperature range and composition range, good selectivity for CO2 over other undesired constituents (such as N2 and O2), good kinetics, high durability, good compatibility, and reasonably low cost. Several adsorbents are potential candidates for CO2 capture. For example, molecular sieves are materials whose atoms are arranged in a lattice or framework in such a way that a large number of interconnected uniformly sized pores exist. The pores generally only admit molecules of a size about equal to or smaller than that of the pores. Molecular sieves, thus, can be used to adsorb and separate or screen molecules based on their size with respect to the pores. One class of molecular sieves is zeolites. Zeolites are hydrated silicates of aluminum and frequently contain cations, which are exchangeable. Zeolites can be naturally occurring or artificial. Naturally occurring types include chabazite, clinoptilolite, erionite, heulandite, and mordenite, to name but a few. Artificial zeolites include, inter alia, types A, D, L, R, S, T, X, Y, ZSM, mordenite, or clinoptilolite. Most specific varieties of those include a numerical designation or the abbreviation of the predominant cation.
Several zeolite candidates for separating carbon dioxide from flue gas were studied by Harlick and Tezel, including zeolites 5A, 13X, NaY, and ZSM-5 (Harlick, P. J. E. and F. H. Tezel, “An Experimental Screening Study for CO2 Removal from N2,” Mesoporous and Microporous Materials, 76, 71-79 (2004)). In addition to those, some types of activated alumina, silica gel, 4A zeolite, and activated carbon are plausible choices, according to the characteristics listed above, but depending on the product specifications, and the operating conditions for a specific application.
“Adsorbent” for present purposes, then, comprehends a porous solid, particulate material or mixture of materials, which selectively admits and retains within its pores (or adsorbs) one or more components from a mixture containing at least one other component. The mixture in this case is a process gas contaminated therewith, such as those adsorbents discussed infra. While the term “adsorbent” will be used often for convenience of description, a porous solid, particulate material, often ranging in size from about 0.1 mm to 10 mm is meant and should be understood by the skilled artisan. Too, use of the term “particulate adsorbent” or “solid adsorbent” also refers to “adsorbent”, as defined herein.